Generally, in any type of wireless communication system, baseband voice or data signals are carried on a radio frequency (RF) carrier frequency. Thus, circuitry must be provided in a transmitter that places the baseband signal on the RF carrier signal. This process is commonly termed frequency up-conversion and the circuitry that performs this process is commonly called a frequency up-converter. Likewise, in the receiver, the baseband signal must be extracted from the incoming RF receive signal picked up by the receive antenna. This process is commonly termed frequency down-conversion and the circuitry that performs this process is commonly called a frequency down-converter.
There are several techniques in common use for performing frequency up-conversion and frequency down-conversion. One such technique is heterodyning. In heterodyning, the conversion between the RF frequency FRF and the baseband frequency FBB (in either direction) is conducted in two steps. That is, taking frequency down-conversion as an example, the received RF signal is first mixed with a first local oscillator (LO1) signal having a fixed frequency FLO1 spaced relatively far away from the RF carrier frequency, e.g., ½ the RF carrier frequency. When two signals such as the RF signal and the LO1 signal are mixed, it produces signals at sideband frequencies of FRF−FLO1 and FRF+FLO1. The output of the mixer is filtered to eliminate the FRF+FLO1 sideband. The remaining signal at the FRF−FLO1 side band is herein termed the intermediate frequency or IF signal at frequency FIF. That signal is then introduced into a second mixer in which it is mixed with a second local oscillator signal (LO2) having the same frequency as the center frequency of the IF signal, namely, FLO2=FIF. Accordingly, the output of the second mixer has a center frequency of FIF−FLO2. Since, as noted above, FIF and FLO2 have the same frequency, FIF−FLO2=0 Hz, i.e., the center frequency of the output of this second mixer is at DC (or 0 Hz). Accordingly, the signal output from this second mixer is at baseband, i.e., only the baseband data signal remains. The baseband signal is then, of course, provided to signal processing circuitry for extracting the data from it.
One of the disadvantages of the heterodyning technique is that it requires two local oscillators and their incumbent circuitry, expense, and complexity.
Thus, it is common today to perform frequency down-conversion from the RF frequency to the baseband in a single step. These techniques are called direct conversion techniques. Direct conversion techniques generally fall into two categories, namely, zero-IF and low-IF techniques.
FIG. 1A is a block diagram illustrating a zero-IF direct frequency down-converter. In this technique, the RF signal is input to a low noise amplifier 101 in order to produce an amplified RF signal. The amplified RF signal is split into two. One branch is fed to a first mixer 103 that mixes it with a local oscillator signal, the local oscillator signal being equal in frequency to the center frequency of the RF signal. The other branch is fed to a second mixer 105 that mixes it with a second local oscillator signal having the same frequency as the first local oscillator signal, but 900 out of phase with it. The outputs of the two mixers 103 and 105 are each passed through low pass filters 107 and 109, respectively, to produce two baseband signals centered at DC and 90° out of phase with each other in order to reject the image frequency at the input of the receiver.
FIG. 1B is a block diagram illustrating a low-IF frequency down-converter for comparison. This technique is similar to the zero-IF technique described above, except that the ultimate output actually is not at DC, but instead is slightly displaced from 0 Hz (e.g., a few MHz) in order to avoid problems associated with DC offset at the output of the mixers and also to eliminate low frequency noise issues. In this technique, the RF signal is input to a low noise amplifier 111 in order to produce an amplified RF signal. The amplified RF signal is split into two. One branch is fed to a first mixer 113 that mixes it with a local oscillator signal, the local oscillator signal being offset slightly in frequency (e.g., a few MHz) from the center frequency of the RF signal. The other branch is fed to a second mixer 115 that mixes it with a second local oscillator signal having the same frequency as the first local oscillator signal, but 90° out of phase with it. The outputs of the two mixers 113 and 115 are each passed through band pass filters 117 and 119, respectively, to produce two baseband signals at a relatively low frequency (e.g., a few MHz) and 90° out of phase with each other.
The steps of zero-IF and low-IF frequency up-conversion are essentially the opposite of those described for zero-IF and low-IF frequency down-conversion.
FIG. 1C is a block diagram of an exemplary zero-IF quadrature direct frequency up-converter. A quadrature baseband signal comprising two components BBI and BBQ are first fed through first and second low pass filters 138, 139. The outputs of the low pass filters are fed to first and second mixers 141, 142, respectively, where they are mixed with the two quadrature components LOI and LOQ of a quadrature local oscillator signal. The frequency of the local oscillator signal is the desired RF transmission frequency. The output of the two mixers 141, 142 are combined in an adder 144. The output of the adder 144 is fed to an amplifier 146. The output of the amplifier 146 is the RF signal forwarded to the antenna for transmission.
The advantages of both zero-IF and low-IF direct conversion is that they require only a single local oscillator having two different phases, rather than two local oscillators at different frequencies. A significant disadvantage of direct conversion, however, is the need for a local oscillator that oscillates all the way up at the FRF frequency (or very close to it, in the case of a low-IF technique). Generally, the higher the frequency at which a circuit must operate, the more complex, power-hungry, and expensive that circuit tends to be.
Sub-harmonic mixers also are known in the prior art. In conventional sub-harmonic mixers, the incoming RF signal is mixed with a local oscillator signal having a very high amplitude relative to the amplitude of the incoming RF signal in order to produce non-linearities. Due to the presence of those non-linearities, the sub-harmonic mixer produces output signals not only at the difference between the local oscillator signal and the incoming RF signal (FLO−FRF), but also at harmonics thereof, such as 2FLO−FRF. Accordingly, by using a sub-harmonic mixer, one may utilize a local oscillator that oscillates at a much lower frequency than the RF signal center frequency and still produce signals at baseband or any desired IF band. For instance, let us assume that it is desired to produce an intermediate frequency signal at 8 GHz from a received RF signal at 24 GHz. Using the principles of sub-harmonic mixing, one can employ a sub-harmonic mixer to mix the RF signal with a local oscillator signal at 8 GHz to produce an output having a sideband component at 8 GHz. Particularly, 8 GHz is 2FLO−FRF. On the other hand, if one used a more conventional mixer, it would require a local oscillator operating at 16 GHz to produce an output signal having a frequency component at 8 GHz (i.e., 24 GHz−16 GHz=8 GHz).
There are wireless communications systems in use today and/or in development that transmit at frequencies of 10 GHz or even 24 GHz or higher. Accordingly, direct frequency up-conversion and down-conversion in connection with these signals would require very high frequency (and therefore very complex, very expensive, and very power-hungry) local oscillators and phase locked loops (PLLs).
Accordingly, it is an object of the present invention to provide an improved method for performing frequency down-conversion.
It is another object of the present invention to provide an improved apparatus for performing frequency down-conversion.